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Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 4326
Keywords: pulse-frequency modulation, PFM, pulse-width modulation, PWM, low-current, isolated dc-dc
supply, dc-dc converter, discontinous operation, standby, green, idle current, PFM controller
Reduce Standby Power Drains with Ultra-LowCurrent, Pulse-Frequency-Modulated (PFM) DC-DC
By: Javier Monsalve Kägi, Senior Member of Technical Staff, Applications
Jose Miguel de Diego, Escuela Técnica Superior de Ingeniería, Bilbao, Spain
Jose Ignacio Garate, Escuela Técnica Superior de Ingeniería, Bilbao, Spain
Mar 19, 2009
Abstract: This article explains how to reduce the level of low-current consumption in isolated DC-DC power
supplies and how to improve the performance of those supplies under no-load conditions. Sensitive to today’s
need for innovative "green" solutions, the discussion especially focuses on ways to extend the battery life of
battery-powered electronic devices and communication-system devices with discontinuous transmission.
This article was also featured in Maxim's Engineering Journal, vol. 65 (PDF, 756kB).
A Japanese version of this article appeared in the EDN Japan Power Supplement, December, 2008.
Today, many industrial systems employ battery-powered sensors and transponders to eliminate expensive
cable installations and to reduce overall system power consumption. These industrial systems typically have
an active mode and a standby mode. In active mode the sensor delivers data to the transponder (a radio
modem) which transmits the data to a host system. In standby mode the transponder and sensor go to sleep
for a fixed or variable time period. This start-and-stop operation, often referred to as a discontinuous operating
mode, maximizes the battery life of the device.
For an application like a watering system that leverages GSM radio modules for the sensors, maintenance
costs would be high if the batteries powering the GSM radios had to be replaced every few days, or even
every few weeks. Since such a system spends most of its time in standby or sleep mode, minimizing the
power drain from the battery when no activity is taking place would go a long way toward extending battery
life. In this system no-load quiescent current becomes a key design consideration, and for safety concerns,
galvanic isolation is an important aspect of the design.
To address these concerns, designers must focus on the design of the DC-DC converter to ensure that it
consumes as little current as possible during no-load conditions. All DC-DC converters, even during standby,
can consume significant quiescent current. One commercial power-supply module (the RECOM® R-78A3.31OR), for example, draws about 7mA under no-load conditions. However, with some attention to topology and
careful design, an isolated DC-DC converter module with a no-load current drain of less than 1mA can be
The 30x difference in current drain can translate into reduced battery replacements. For example, even if the
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system's batteries are rechargeable, then additional recharge cycles might be needed if the higher currentdrain supply is used. Moreover, batteries that are recharged often, wear out sooner and end up in landfills.
Similarly, if the device employs one-time-use batteries, they will discharge sooner with a higher standby
current and get discarded more frequently.
While there are several approaches to the challenge, this article looks at the use of pulse-frequency
modulation (PFM) to achieve a 1700:1 ratio between the device's on and standby states.
System Characteristics
Typical power consumption versus time looks like the graph in Figure 1. Here the load current spikes during
operation or active charging, and then drops when the device is idle. The idle current, IZ, must be minimized
to reduce battery drain and extend battery life and standby time. Thus, the isolated DC-DC converter needs
ultra-low-current consumption when no load is connected, and should also provide high isolation from input to
output. Ideally, the converter should also offer high-conversion efficiency and a small footprint.
Figure 1. The relationship between the on and standby states of a communication device with discontinuous
The typical commercial DC-DC converters listed in Table 1 show input currents of 7mA to 40mA when no load
is connected with an input of 12V. These converters traditionally employ pulse-width-modulation (PWM)
controllers. However, PWM controllers always have an active oscillator, even when there is no load, and that
oscillator continually draws current from the battery.
Table 1. Characteristics of Commercial DC-DC Converters
(A) (IOUT = 0, mA) (%)
Traco® Electronic AG
TEN 5-1210 12
XP Power
JCA0412S03 12
RECOM International Power RW-123.3S
C&D Technologies®
HL02R12S05 12
Bourns® Inc.
MX3A-12SA 12
93 12
81 RECOM International Power R-78A3.3-1
A PFM Controller Topology
An alternative approach is to use a DC-DC converter that employs a pulse-frequency-modulation (PFM)
controller.¹ A PFM controller uses two one-shot circuits that only work when the load drains current from the
DC-DC converter's output. The PFM is based on two switching times (the maximum on-time and the minimum
off-time) and two control loops (a voltage-regulation loop and a maximum peak-current, off-time loop).
The PFM is also characterized by control pulses of variable frequency. The two one-shot circuits in the
controller define the T
(maximum on-time) and the T
(minimum off-time). The T
one-shot circuit
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activates the second one-shot, TOFF. Whenever the comparator of the voltage loop detects that VOUT is out of
regulation, the TON one-shot circuit is activated. The time of the pulse is fixed up to a maximum value. This
pulse time can be reduced if the maximum peak-current loop detects that the current limit is surpassed.
The quiescent current consumption of a PFM controller is limited only to the current needed to bias its
reference and error comparator (10s of µA). In contrast, the internal oscillator of a PWM controller must be
turned on continuously, leading to a current consumption of several milliamps. The implementation presented
in this article keeps the current consumption to less than 1mA at 12V by using a PFM controller topology.
Field systems such as the watering system must endure harsh environments, and thus the DC-DC converter
in those systems should be galvanically isolated. A transformer provides the isolation, but the challenge is to
feed back the voltage reference from the secondary side to the primary side without breaking the isolation.
The most common approach solves the problem by using either an auxiliary winding or an optocoupler.
The power-supply topology is a step-down approach; the battery pack used by the application has a nominal
voltage of 12V, while the internal electronic circuits in the system operate at 3.6V, nominal. Figure 2 shows
the schematic diagram of the DC-DC switching regulator and the bill of materials with component values is
provided in Table 2. When the control loop is regulating the voltage, the optocoupler requires a constant
current through the LED on the primary side of the transformer. The lower limit of the current is fixed by the
optocoupler's CTR at low bias currents (63% at 10mA, and 22% at 1mA) and by a reduction of the response
time (2µs at 20mA and 6.6µs at 5mA).
Figure 2. Schematic of an isolated PFM flyback DC-DC converter.
Table 2. Component Bill of Materials for PFM Flyback DC-DC Converter
Reference Values
470µF 25V
CEL 470µF, 25V, +105°C, 10mm x
10mm SMD
UUD1E471MNL1GS (Nichicon®)
CS 180p C COG, 50V 0603/1
GRM39 COG 181 J 50 PT (Murata®)
C1, C4,
100nF 16V
#CSMD 100nF K X7R 16V 0603/1
GRM39X7R104K16PT (Murata)
C5, C8
100µF 16V
CEL TAN 100µF ±20% E 16V 0.1Ω
T495D107K016ATE100 (Kemet®)
CS 100p C COG 50V 0603/1
GRM39 COG 101 J 50 PT (Murata)
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1nF 50V
#CS 1n M X7R 50V 0603/1
GRM39 COG 271 J 50 PT (Murata)
CS 150p C COG 50V 0603/1
GRM39 COG 151 J 50 PT (Mutata)
MBRS230LT3G D Schottky 2A, 30V SMB
MBRS230LT3G (ON Semiconductor®)
MBRA160T3G D Schottky 1A, 60V SMA
MBRA160T3G (ON Semiconductor)
22µH 1.2A
L SMD 22µH, 1.2A, 0.19Ω
SRR0604-220ML (Bourns®)
Q IRFR120 DPAK 8.4A, 100V, 0.270Ω,
IRFR120 (Int. Rectifier)
R1, R6
RS 680R J 1/16W 0603/1
RK73B 1J T TD 680 J (KOA Speer®)
R9, R2
#RS 100K F 1/16W 0603/1
RK73H 1J T TD 1003 F (KOA Speer)
#RS 10R J 1/16W 0603/1
RK73B 1J T TD 100 J (KOA Speer)
#RS 4K7 J 1/16W 0603/1
RK73H 1J T TD 4701 J (KOA Speer)
#RS 390K F 1/16W 0603/1
RK73H 1J T TD 3903 F (KOA Speer)
RS R047 J 1206 /1
SR73 2B T TD R047 J (KOA Speer)
RS 270K F 0603 /1
RK73H 1J T TD 2703 F (KOA Speer)
RS 820K F 0603 /1
RK73H 1J T TD 8203 F (KOA Speer)
#R SMD 100R-J 1206/1
RK73B 2B T TD 101 J (KOA Speer)
EP10 3F3
CSHS-EP10-1S-8P-T (Ferroxcube®Nuctor)
DC-DC controller
Maxim Integrated Products
U TLV431A V.REF 1.25V SOT23-5
#U SFH6106-2 OPTO 63-125%, 5.3kV
SFH6106-2 (Vishay®)
The current consumption of the output voltage-divider (formed by resistors R5 and R11) is fixed to 7µA.
Because of this, the 0.5µA required by the reference input plus its thermal deviation does not significantly
affect the output voltage. Additionally, the voltage measured at the divider output does not suffer a relevant
delay, thanks to the low-input capacitance. This latter fact precludes the need for a capacitive divider to
reduce the input capacitance of the precision reference. In the optocoupler, the phototransistor draws 60µA
(|I FB | < 60nA), which translates into a current flow through the LED of less than 230µA (CTR ~26%).
Controlling It All
To implement a PFM controller, the MAX1771 BiCMOS step-up, switch-mode power-supply controller (U1)
can be used to provide the necessary timing. The MAX1771 offers improvements over prior pulse-skipping
control solutions: reduced size of the inductors required, due to a 300kHz switching frequency; the currentlimited PFM control scheme achieves 90% efficiencies over a wide range of load currents; and a maximum
supply current of just 110µA. Besides these advantages, the main characteristics of the MAX1771 in a
nonisolated application are: 90% efficiency with load currents ranging from 30mA to 2A; up to 24W of output
power; and an input-voltage range of 2V to 16.5V.
The resistances of the voltage-control loop have been chosen to have the highest possible values. This
decision represents a trade-off between current consumption and loop stability. As a result, the current
through the voltage-divider is less than 7µA. Since the filtering capacitors are nonideal, capacitor leakage
current must be added to this current. In this design, filter capacitor-leakage current in C5 and C8 is less than
20µA. If lower leakage is required, these caps could be upgraded to ceramic capacitors with the following
characteristics: 100µF, 6.3V, X5R, and 1206 size (Kemet C1206C107M9PAC). Using ceramic capacitors
reduces the capacitor leakage to just a few microamps. Note, however, that the ceramic capacitors cost about
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3x that of the tantalum capacitors, and that difference would increase the system cost.
Figure 3 shows the prototype PFM DC-DC converter that draws a quiescent current of just 0.24mA. The
board measures less than 50mm by 30mm, can deliver 3.6W with an input-voltage range of 10V to 15V (12V
nominal), and operates at a switching frequency of 300kHz. The converter can supply a maximum constant
output current of 1A while delivering a regulated output of 3.6V. Employing a flyback topology (step down) with
both current and voltage feedback control, the converter output is galvanically isolated from the input.
More detailed image (PDF, 4.59MB)
Figure 3. Top view of the DC-DC PFM converter prototype for wireless applications.
The prototype can be used in various wireless applications that operate in a discontinuous transmission mode.
The current consumption of the modules can peak at 3A, and the maximum mean current is 1A. To reduce
the current peaks and avoid the problems that they generate in the performance of the radio, the techniques
described in references 2 and 3 are used. Additionally, some basic guidelines suggest that designers should
use high-value capacitors that have low series resistances.
Qualifying Design Performance
To verify the performance of the power supply, the following parameters are measured: the input voltage, VIN;
the input current, I IN; the nominal output voltage, VOUT ; the load current consumption, IOUT ; and the
efficiency of the power supply. Tables 3 and 4 show the measurement results, including the losses on the
common-mode input filter and the losses of the protection circuitry. It is also important to remember that
power supplies handling low power levels are not as efficient as power supplies handling higher loads. The
higher-load power supplies are usually synchronous, which helps to reduce the losses in the active devices.
Table 3. Current Consumption Under a No-Load State for Different Input Voltages
(V) (mA) (V)
10.0 0.244 3.615 0
12.0 0.239 3.615 0
15.0 0.227 3.615 0
The current consumption of the power supply with a PFM control scheme has been reduced to 0.24mA. Due
to component values selected, however, the control loop may oscillate during certain load conditions. To
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prevent self-oscillation, designers must account for the various tolerances of the components in a production
environment. Thus, the values of the resistors and capacitors used in the loop must be selected with care.
Table 4 provides the values for the input and output parameters of the power supply at various load
conditions. The optimum efficiency is reached at normal conditions and within the nominal load range.
Table 4. Efficiency at Nominal Voltage for Different Loads
VOUT IOUT Efficiency
(V) (mA) (V)
(A) (%)
12.0 0.24 3.615 0
12.0 61
3.615 0.14 69.14
12.0 83
3.615 0.2
12.0 121
3.615 0.3
12.0 160
3.615 0.4
12.0 200
3.615 0.5
12.0 240
3.615 0.6
12.0 281
3.615 0.7
12.0 323
3.615 0.8
12.0 367
3.615 0.9
12.0 411
3.615 1.0
The efficiency of the DC-DC converter with no load is represented as zero (Figure 4), because the current
consumed by the wireless device in standby mode and referred to the 3.6V output side is below 140µA. This
current is negligible when compared to the 0.24mA of the power supply's input-current consumption under noload conditions.
Figure 4. Efficiency of the power supply for different load conditions at the input nominal voltage (12V).
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Figure 5a. Output voltage and control voltage without load (10ms/div, CH1 1V/div, and CH2 5V/div).
Figure 5b. Output voltage and control voltage for 0.1A load (20ms/div, CH1 1V/div, and CH2 5V/div).
Figure 5c. Output voltage and control voltage for 0.5A load (20ms/div, CH1 1V/div, and CH2 5V/div).
The waveforms in Figures 5a, b, c, and d show the output voltage and control voltage for various loads; the
control pulses at the gate of the switching device become more frequent as the load increases. The converter
prototype shows the signals at no load, 100mA, 500mA, and 1A current loads. The scope traces graphically
illustrate the operation of the PFM control scheme. The lower scope trace is scaled by 5x to make it more
visible. The X axis represents the time and the Y axis the voltage.
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Figure 5d. Output voltage and control voltage for 1A load (20ms/div, CH1 1V/div, and CH2 5V/div).
Initial industry surveys indicate that the best commercial isolated DC-DC converters for power supplies with
low current consumption under no-load conditions typically have about 20mA minimum current consumption.
With minimal effort, however, designers can use a PFM scheme to implement a low-IQ , isolated power supply
that has the lowest current consumption on the market. The no-load current consumption of the power supply
presented here is only 0.24mA.
1. Maxim Integrated Products application note 664, "Feedback Isolation Augments Power-Supply Safety
and Performance," and EDN magazine (June 19, 1997).
2. J. Ig. Garate, J. M. de Diego, "Consequences of Discontinuous Current Consumption on Battery Powered
Wireless Terminals" [ISIE06, Paris, France, Oct. 2006].
3. J. M. de Diego, J. Ig. Garate, "Improvements of Power Supply Systems in Machine to Machine Modules
and Fixed Cellular Terminals with Discontinuous Current Consumption" [Digests 9 th ICIT06, Mumbai,
India, Dec. 2006].
Additional Reading
1. I. Haroun, I. Lambadiris, R. Hafez, "RF System Issues in Wireless Sensor Networks," Microwave
Engineering Europe (Nov. 2005), pp. 31–35.
2. J. P. Joosrting, "Power dissipation could limit smartphone performance," Microwave Engineering Europe
(Apr. 2006), comment p. 9. Available at:
3. "MAX1649/MAX1651, 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC
Controllers," Maxim Integrated Products Data Sheet, 19-0305; Rev 2; 9/95.
4. "MAX1771, 12V or Adjustable, High-Efficiency, Low I Q , Step-Up DC-DC Controller," Maxim Integrated
Products Data Sheet, 19-0263; Rev 2; 3/02.
5. J. Ig. Garate, J. M. de Diego, J. Monsalve, "Ultra Low Input Current Consumption Power Supplies"
[ISIE07, Vigo, Spain, Jun. 2006].
6. J. Ig. Garate, J. M. de Diego, J. Monsalve, "Sistemas de transmisión discontinua. FAC aisladas y de muy
bajo consumo en vacío," Mundo Electrónico (Oct. 2007), pp. 38–45.
7. R. W. Erikson, Fundamentals of Power Electronics, 1 st Ed. (Chapman and Hall, New York, 1997).
8. B. Arbetter, R. Erikson, and D. Maksimovic, "DC-DC converter design for battery-operated systems,"
Proceedings of IEEE® Power Electronic Specialist Conference (1995), pp. 102–109.
9. B. Sahu and G. A. Rincora, "A Low Voltage, Non-Inverting, Dynamic, Synchronous Buck-Boost Converter
for Portable Applications," IEEE Transactions on Power Electronics, vol. 19, no. 2 (Feb. 2004), pp. 443–
10. G. A. Rincora and P. E. Allen, "A Low-Voltage, Low Quiescent Current, Low Drop-Out Regulator," IEEE
Page 8 of 9
Journal of Solid-State Circuits, vol. 33, no. 1 (Jan. 1998), pp. 36–44.
11. D. Maksimovic, "Power management model and implementation of power management ICs for next
generation wireless applications," Tutorial Presented at the International Conference on Circuits and
Systems [ISCAS, 2002].
12. Data Acquisition Linear Devices Databook. Vol. 3, National Semiconductor Corporation (1989).
13. "TPS62110 TPS62111 TPS62112, 17-V, 1.5-A, Synchronous Step-Down Converter," Texas Instruments
Incorporated, SLVS585–JULY 2005 (2006).
Bourns is a registered trademark of Bourns, Inc.
C&D Technologies is a registered trademark of C&D Technologies, Inc.
Ferroxcube is a registered trademark of Ferroxcube International Holding B.V.
IEEE is a registered service mark of the Institute of Electrical and Electronics Engineers, Inc.
Kemet is a registered trademark of KRC Trade Corporation.
KOA Speer is a registered trademark of KOA Speer Electronics, Inc.
ON Semiconductor is a registered trademark and registered service mark of Semiconductor Components
Industries, L.L.C.
RECOM is a registered trademark of Recom Electronic GmbH Ltd.
Texas Instruments is a registered trademark and registered service mark of Texas Instruments Incorporated.
TRACO POWER is a registered trademark of Traco Power Ltd.
Vishay is a registered trademark of Vishay Intertechnology, Inc.
Related Parts
5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, StepDown DC-DC Controller
Free Samples MAX1651
5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, StepDown DC-DC Controller
Free Samples MAX1771
12V or Adjustable, High-Efficiency, Low IQ , Step-Up DC-DC
Free Samples MAX8515A
Wide-Input 0.6V Shunt Regulators for Isolated DC-DC
Free Samples More Information
For Technical Support:
For Samples:
Other Questions and Comments:
Application Note 4326:
APPLICATION NOTE 4326, AN4326, AN 4326, APP4326, Appnote4326, Appnote 4326
Copyright © by Maxim Integrated Products
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